title: Synthetic peptide helices in crystals: structure and antiparallel and skewed packing motifs for alpha-helices in two isomeric decapeptides.
abstract: The isomeric decapeptides Boc-Aib-Ala-Leu-Ala-Aib-Aib-Leu-Ala-Leu-Aib-OMe (II) and Boc-Aib-Ala-Aib-Ala-Leu-Ala-Leu-Aib-Leu-Aib-OMe (III), are predominantly alpha-helical with little effect on the conformation with interchange of Aib/Ala residues or Aib/Leu residues. The packing motif of helices in crystal II is antiparallel, whereas the helices pack in a skewed fashion in crystal III, with a 40 degrees angle between neighboring helix axes. Crystal III contains a water molecule in a hydrophobic hole that forms hydrogen bonds with two carbonyl oxygens that also participate in 5----1 type hydrogen bonds. Values for helical torsional angles phi and psi assume a much wider range than anticipated. Crystal II: C49H88N10O13, space group P2(1), with a = 16.625 (2) A, b = 9.811 (5) A, c = 18.412 (3) A, beta = 99.79 (1) degrees, Z = 2, R = 5.7% for 4338 data with magnitude of F0' greater than 3 sigma(F). Crystal III: C49H88N10O13 x 1/2H2O, space group P2(1) with a = 11.072 (2) A, b = 34.663 (5) A, c = 16.446 (3) A, beta = 107.85 (1) degrees, Z = 4, R = 8.3% for 6087 data with [F0[ greater than 3 sigma(F).
authors.affiliation: Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, DC 20375-5000.
authors.forename: I L
authors.initials: IL
authors.lastname: Karle
authors.forename: J L
authors.initials: JL
authors.lastname: Flippen-Anderson
authors.forename: K
authors.initials: K
authors.lastname: Uma
authors.forename: P
authors.initials: P
authors.lastname: Balaram
mesh_terms: D000595:Amino Acid Sequence; D003460:Crystallization; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009842:Oligopeptides / Q000737:chemistry*; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D009842:Oligopeptides